Rohit Sharma averages 88.33 in 14 home Tests and 26.32 in 18 away games - that's a difference of 62.01, which is easily the record, among those who had at least ten innings both home and away. Marnus Labuschagne currently lies second - he averages 85.42 in Australia and 39.45 away, a difference of 45.96.
In that match, Aiden Markram and Farhaan Behardien came together in the 13th over, with the Titans reeling at 31 for 5 against the Cape Cobras in Cape Town in March 2019. They weren't separated until Markram fell in the final over, after a partnership of 272 - which was indeed a record for the sixth wicket in all List A cricket. It beat the unbroken stand of 267 by Grant Elliott and Luke Ronchi (who thumped 170 not out from 99 balls) for New Zealand against Sri Lanka in Dunedin in 2015, which remains the record for one-day internationals. There have been eight further partnerships of 200 or more in List A matches, including two further instances in 2019.
You're right to say that Angelo Mathews has reached 50 on ten occasions in Tests in Galle without going on to three figures - his highest score there is 95 not out, against Australia in August 2011. Surprisingly, though, it's not the record: Australia's Allan Border reached 50 at the Sydney Cricket Ground on 11 occasions, but his highest scores there were 89 and 83, against England in 1983.
You're right that no one has yet finished with an individual score of 229 in a Test match - the next unmade scores are 238, 252 and 265. The lowest score not yet made in one-day internationals is 155 (weirdly, there have been six 154s and six 156s), followed by 165 and 182. And in T20Is, no one has yet finished his innings on 108, 112 or 115.
India's vaunted quartet of spinners back then was made up of Bishan Bedi, who finished with 266 wickets in 67 Tests, Bhagwath Chandrasekhar (242 in 58), Erapalli Prasanna (189 in 49) and S Venkataraghavan (156 in 57).
Steven Lynch is the editor of the updated edition of Wisden on the Ashes